1. Field of the Invention
The present invention generally relates to mining and construction tools and, more particularly, is concerned with an annular or cylindrical shim with multiple perforations for joining a conical carbide tip to the body of a conical bit useful in coal mining.
2. Description of the Prior Art
Considering that thermal expansion rates of hard carbide and tungsten alloys very from one-third to one-half that of steel, over the years brazing has proven to be an extremely successful and advantageous method of mounting carbides to steel. While it is most satisfactory when applied to small-area, short-length joints, it can also be satisfactorily applied to larger joints. However, whereas small joints are produced by relative simple, straightforward construction methods, large joints normally require more complicated methods to avoid difficulties which would otherwise be encountered in construction of the large joints.
These difficulties stem from the amount of thermal or brazing strain (or stress) created in the joined parts during formation of the braze joint. The amount of brazing strain created in small areas or short lengths of parts as a braze joint between them solidifies is usually minor and normally absorbed without materially weakening the joined parts. On longer parts or larger areas, however, the brazing strain becomes proportionately greater and can lead to considerable bending and frequently to fracture of the carbide part. Thus, other methods of construction have been used heretofore to relieve brazing strains induced in carbide and steel parts by these longer or larger braze joints.
One method of construction to relieve brazing strain is to use a sandwich braze joint composed of a shim of a malleable metal, such as copper, between the carbide and steel parts. The malleable metal shim does not melt during brazing of the joint will deform under the brazing strain without losing its bond to the steel or carbide parts, thus relieving the stress or strain to a large degree. Representative examples of prior art shims include those disclosed in U.S. Pats. Nos. to Braun (1,956,233), Orr et al (4,330,004) and Dziak (4,356,873); Swiss Pat. No. to Braun (227,635); German Pat. Application No. to Hasewend (B 28706Ib/49h); and British Pats. Nos. 386,909, 664,983 and 668,810.
In conical bits which utilize conical carbide tips, heretofore a cylindrical recess has normally been provided in the forward end of the bit body in which the conical tip is seated and brazed to the bit body at the sidewall of the recess. This approach has been satisfactory historically for relatively small diameter carbide tips. However, carbide tips are currently being provided in larger diameter sizes. As their diameter increases, the size of the recess in the bit body must also increase leaving less metal at the forward end of the bit surrounding the tip to absorb the stresses arising in the metal during formation of the braze joint.
Some alternative technique must be devised to reduce the formation of stress in the case of conical bits using larger diameter conical carbide tips. However, none of the above-cited patents appear to disclose an optimum alternative approach. Consequently, a need exists for an alternative technique to produce a more effective braze joint between the conical bit body and carbide tip so as to provide a more durable tool having an increased useful life.